Social isolation and loneliness both have to do with social embeddedness, describing them, respectively, as an objective fact and as a subjective experience of the individual. There is a lack of theoretical elaboration of the concept of social isolation, and often, it is defined and measured ad hoc. Loneliness is a well-developed theoretical concept, and measurement tools with good measurement properties are available. The outcomes and determinants of social isolation and of loneliness are only partially consistent and the correlation is low. The two concepts present different views of people’s lives, indicate different vulnerabilities, and should be treated in their own way.